1. Field of the Invention
The invention relates to removal of tubular tissue supports (stents) from hollow organs of humans and/or animals.
2. Description of the Related Art
A stent (medical technology) is an implant which is introduced into hollow organs (e.g. into veins or arteries, bile ducts, the trachea or the esophagus) in order to brace the wall radially outwards. Examples of use of stents are in coronary vessels for prophylaxis of restenosis after PTCA (percutaneous transluminal coronary angioplasty).
Stents are small grid structures in the form of a tube composed of metal or of polymers, often used in the context of angioplasty, in which strictures in vessels are widened. In cancer treatment, stents serve to prevent closure of strictures caused by malignant tumors in respiratory passages, bile ducts or the esophagus, after these have been expanded.
Stents are usually cylindrical products composed of a type of wire mesh (wire coil design) or of tubes, which may be perforated or unperforated (slotted tube design). The length of commonly used stents is from 1 to 12 cm, their diameter being from 1 to 12 mm.
A stent is subject to various requirements. First, the support has to exert large radial forces on the hollow organ requiring support. Second, the support must be able to be compressed radially to permit its easy introduction into a hollow organ without at the same time injuring the vessel wall or the surrounding tissue.
In order to meet the above requirements, the stents are used in compressed form and not expanded until the correct location has been reached. In the compressed condition, the diameter is markedly smaller than in the expanded condition. In principle, this procedure can also be utilized for minimally invasive removal of the stent. However, one possible problem here is that the metallic materials usually used are riot always capable of entirely uniform expansion and recompression, and there is a resultant potential risk of injury to adjacent tissue.
Two different technologies are used for minimally invasive stent use: (1) expandable-balloon stents (system composed of balloon, catheter, stent); and (2) self-expandable stents (system composed of introductory sheath (protective sheath), catheter, stent).
Self-expanding stents are generally composed of shape-memory materials (SM material). Shape-memory materials are materials which change their external shape on exposure to an external stimulus. The materials are, by way of example, capable of controlled change in their shape when the temperature is increased above what is known as the switching temperature (Ttrans). The shape-memory effect is utilized for “spontaneous” enlargement of the diameter of the stent, and to fix the stent at the location of use.
The shape-memory effect is not a specific property of any of the materials. Rather, it is a direct result of the combination of structure and morphology and of a processing/programming technology.
In shape-memory materials, a distinction is made between a permanent and a temporary shape. The material is first converted to its permanent shape, using conventional processing methods (e.g., extrusion). The material is then converted, reshaped and fixed into its desired temporary shape. This procedure is also termed programming. It is composed either of heating of the specimen, reshaping and a cooling procedure, or else of the shaping at relatively low temperature. The permanent shape has been held in memory, while the temporary shape is actually present Heating of the material to a temperature higher than the transition temperature for a change of morphology (switching temperature) triggers the shape-memory effect and thus causes resumption of the permanent shape held in memory.
The shape-memory effect, which permits controlled alteration in the shape of a material by application of an external stimulus is described, by way of example, in the overview articles “Shape Memory Alloys”, Scientific American, vol. 281, 74-82 (1979) and Angew. Chem., 114, 2138-2162 (2002).
An example of a metallic SM material used is nitinol, an equiatomic alloy composed of nickel and titanium (J. Appl. Phys., 34, 1475 (1963)). However, nitinol cannot be used when nickel allergy is present. The material is moreover very expensive and programmable only by complex methods. This programming process needs comparatively high temperatures, and programming in the body is therefore impossible. The SM material is therefore programmed outside the body, (i.e. converted to its temporary shape). After implantation, the shape-memory effect is then triggered and the stent is expanded, i.e. regains its permanent shape. Removal of the stent by again utilizing the shape-memory effect is then impossible. Another frequent problem with metallic stents, not only in the vascular sector, is occurrence of restenosis.
In contrast, other metallic stents composed of SM materials, for example those described in U.S. Pat. No. 5,197,978, also permit utilization of the shape-memory effect for stent removal. However, production of these metallic materials is very complicated and tissue compatibility is not always ensured. Inflammation and pain patterns occur because of the poor matching of the mechanical properties of the stent.
The temporary stent described in U.S. Pat. No. 5,716,410 is a spiral composed of a polymeric shape-memory material (SMP). The SMP material includes an embedded heating wire. The heating wire has connection by way of a catheter shaft to an electrical control unit, the end of the shaft taking the form of a hollow tube pushed over one end of the spiral. If the implanted stent in its expanded, temporary shape is heated above the switching temperature Ttrans, the diameter of the spiral decreases. The intention is that this permits easy removal of the stent. A disadvantage of the spiral structure is that the radial forces are too small to expand tubular cavities. The radial forces of the spiral are distributed merely over a very small area of contact with the tissue; there is the danger of local mechanical pressure-overloading, and indeed in some instances incision into the tissue. Furthermore, it is difficult to secure the catheter shaft (heating element) to the heating wire of the implanted spiral because first the catheter shaft has to be pushed over one end of the spiral.
U.S. Pat. No. 5,964,744 describes implants, such as tubes and catheters, for the urogenital sector or gastro-intestinal tract composed of polymeric shape-memory materials which comprise a hydrophilic polymer. In an aqueous medium the material absorbs moisture and thus softens and changes its shape. The material can also soften on heating. In the case of the ureteral stent, the effect is utilized in order to flex the straight ends of the stent at the location of use (e.g. kidneys and bladder). The result is to fix the ureteral stent at the location of use, so that the stent cannot slip during peristaltic movements of the tissue.
WO 02141929 describes tubular vessel implants with shape memory which are also suitable, by way of example as bile duct stents. The material is an aliphatic, polycarbonate-based thermoplastic polyurethane with biostability.
The shape memory polymer stents known from DE 10357747 and DE 10357744 do not need to be removed from the body because of their biodegradability. The materials for the stents are described therein to be elastic only when heated above the transition temperature Ttrans of the polymer.
DE 10357743 and DE 10357742 describe temporary shape memory polymer stents, that are heated above the transition temperature Ttrans of the polymer to change their shape to a smaller form before their removal.
U.S. Pat. No. 6,245,103 describes bioabsorbable, self-expanding stents composed of braided filaments. Here, a stent is compressed by applying an external radial force. The stent has been mounted on a catheter and is held in stressed, compressed condition by an outer sheath. When the stent is expelled from this arrangement its diameter spontaneously enlarges because of the resilience of the elastic material. This change is not the shape-memory effect, which is triggered by an external stimulus, (e.g., a temperature increase.
Removal of an expanded stent, as indicated above, is difficult. When the stent has to be withdrawn from a tubular cavity there is a risk that the surrounding tissue will be injured by abrasion in the process, because the stent is too large and has sharp edges. The shape-memory effect is, therefore, also used again to reduce the diameter of the stent when the stent is in turn to be removed. Examples of removable stents composed of metals with shape-memory properties are known (See, e.g., U.S. Pat. Nos. 6,413,273; 6,348,067; 5,037,427 and 5,197,978.